Medical diagnoses utilizing an electronic endoscope have widely been practiced in the medical field these days. The electronic endoscope has a solid state imaging device like a CCD, which is built in an end of an elongated probing portion that is introduced into a test body, so that the CCD takes an image signal from an internal body site. The image signal is processed in a processor, to display an image of the internal body site on a monitor.
In the field of medical diagnosis utilizing the electronic endoscope, an imaging method called Narrow Band Imaging (NBI) is in the limelight, wherein light of a narrow wavelength band is projected toward a target body part to inspect, to produce an image from reflected light. The image captured this way is called a spectral image, for discrimination from a general image that is produced from white light reflected from the target body part. The NBI method makes it easy to obtain such images that facilitate finding out lesions, e.g. an image enhancing blood vessels in a sub-mucosal layer, and an image enhancing structure of an internal organ, like a stomach wall or surface tissues of intestines, without the need for spraying some pigment on the target part or injecting some contrast agent like indocyanine green (ICG).
Also a technique has recently been brought into practice, wherein a spectral image is obtained from a general image as produced from white light reflected from the target body part, by subjecting the general image to linear approximation through dimension reduction using principal component analysis, or spectral estimation such as Wiener estimation.
For the sake of improving accuracy of spectral estimation, it is effective to increase the number of spectral sensitivity bands from the three bands for red, green and blue of the solid state imaging device. This is called band-multiplication of spectral sensitivity, and a lot of methods for the band multiplication have conventionally been suggested in the field of electronic endoscope and other fields as well. In an example of the conventional methods, as disclosed in JPA 2002-296114, two kinds of band pass filters are alternatively set in front of a lens that forms an image of a subject on photographic film, to capture a pair of images of the same subject through the respective filters, and a spectral image is produced from the images of one pair.
According to another method as suggested in JPA 2005-223700, incident light is separated through a color separation prism into a plurality of light beams of different wavelength bands, and the separated light beams are captured by a corresponding number of solid state imaging devices placed at exit surfaces of the color separation prism, so that a spectral image is produced from the captured images.
JPA 2005-260480 suggests an imaging apparatus, wherein a branching optical system is disposed between an image-forming optical system for forming an image of a subject and a single-chip color imaging device, so that the branching optical system divides a luminous flux of the subject image into a plurality of light beams, and converges the divided light beams individually onto divisional image forming surfaces.
The above three prior arts need the color separation prism, the filters or the branching optical system to be disposed between the imaging lens and the imaging device. Therefore, in order to apply these prior arts to the electronic endoscope, it is necessary to dispose the color separation prism, the filters or the branching optical system beside the solid state imaging device in the tip of the probing portion of the electronic endoscope. Then, the probing portion should be broader than conventional, and it would increase the load on the patients.